While genetic modification of mice has resulted in the generation of transgenic lines that have helped elucidate the role of a myriad of molecules in biological and pathological processes, the same is not true in swine. Technically, the mouse can be genetically manipulated not only inexpensively but also wiyh exquisite precision. Xenotransplantation of pig organs into humans has the potential to significantly alleviate the problems associated with organ donor availability. Unfortunately, before swine can be used as an organ donor for humans it will be necessary to diminish or abolish all or most rejection phenomena including hyperacute rejection, acute vascular rejection, cellular rejection and chronic rejection. This will require not only a thorough undestanding of the rejection phenomena in xenotransplantation, but equally important a series of complex genetic modifications to modify swine molecules involved in each of the rejection steps. We propose here the continuation of the development of technology that will allow such precise genetic manipulation, and its application to the abolishment of hyperacute and acute vascular rejection by inactivation of the main gene responsible for these rejection phenomena, the alfa 1, 3 galactosyl transferase gene (1,3 GT). To reach this overall goal we propose to (a) Develop mechanism for enhancing homologous recombination in somatic cells. (b) Extend our abilities to do complex genetic modifications in somatic cells by extension of cellular life span. (c) Generate cloned swine using non-transgenic and genetically modified fetal fibroblasts. (d) Develop a 1,3 Galactosyltransferase deficient pig. Completion of the proposed aims will result not only in the generation of swine that can be used to ficilitate the develoment of xenotransplant organs, but equally important it will result in the development of technology to allow the application of precise genetic manipulations to swine.